A few weeks ago I dyno'd my car with just a JB4 on map 7 running on E30 and made 529 rwhp/531rwtq. It is the red line in the dyno graph below.
Data Log here:
If you look at the Dyno you will see the blue line dyno makes more torque but power is about the same. That is my map 6 that has a more aggressive boost curve (534rwhp/550RWtq). It comes up to 26 psi and holds it to redline. Map 7 hits 25 psi and tapers a bit towards redline. As you can see even though it is running more boost, it makes the same amount of power once you cross @ 5300 rpm. At this point there is so much back pressure in the turbine that no matter what amount of boost you run it will make the same or even less power due to the turbo being so far out of its efficient operating area.
I had a set of Dp's at the house and was curious if they would make any power at this level. Based on the dyno results and logs from the JB4 it looked like the turbines were the real restriction due to their small size, not the downpipes. My educated guess was there wasn't any more real hp to be found without changing the turbos or running higher octane fuel/meth that would allow higher timing to get the last bit of power out of these turbos.
I installed the DP's (Shifteck's) and went back to the dyno. I would have loved to do this on the same day but the real world and having a life prevented that. The conditions were the same, mid 90's temps on the same dyno with the same correction and same smoothing value. I made sure I was still running E30 and even checked the tire pressures to control the conditions as much as possible.
Here is the Map 7 dyno over laying the previous Map 7 dyno. As you can see there no gain in horsepower with a bump in torque before the magic 5300 rpm where the small turbines kill the power.
Here is the datalog:
I was curious to see if the downpipes would help if I really pushed the turbos and used map 6 to run 27 psi to determine if the downpipes allowed more power up top. As you can see in the dyno, below 5300 the extra boost provided about 10 more horsepower and more torque but actually less power up top, even with the extra boost.
And datalog:
So, in conclusion it looks like on a E30 tune the downpipes don't provide any real power gains. There is a bump in mid range torque but nothing up top where we really need it. On the other hand with a pump gas tune (91/93 octane) the downpipes add power as we have seen in other dynos.